1. Field of the Invention
The present invention relates in general to a laser vibrometer for the detection of acoustic signatures propagated through air and other media, and more particularly to such a system capable of enhanced resolution of such signatures.
2. Discussion of the Prior Art
Vibrometer technology involves the detection and analysis of pressure waves, such as acoustic waves or water waves, that might bear information regarding agitation sources of interest to the observer. Conventional microphones are capable of detecting such waves with varying degrees of accuracy and resolution satisfactory for general applications. All microphone-like devices and technologies possess a pressure-sensing interface, including but not limited to, a diaphragm that receives the incoming acoustic pressure waves and conform its physical motion to mimic that of the incident acoustic, i.e., pressure, waves. In conventional microphones, additional mechanical parts are in general connected to the diaphragm so as to convert the motion of the diaphragm into signals of electrical nature that allow further processing and applications. Such auxiliary mechanical parts might include an electrically conducting rod to induce alternating electrical currents that approximate the motion of the diaphragm, and hence the incoming pressure waves, or alternatively, to induce a capacitance which subsequently leads to a measurable electrical current. Unfortunately, such auxiliary mechanical parts add significant weight to the assembly, and alter/limit the resultant frequency response towards the lower end. Furthermore, such added weight also negatively impacts the sensitivity of the diaphragm assembly in detecting the incoming pressure waves, e.g., acoustic waves, due to the fact that such mechanical parts have innate inertia which can only be overcome by larger amplitude pressure waves, to move and generate detectable output signals.
A more modern alternative, as disclosed in U.S. Pat. Nos. 4,554,836 and 5,883,715, involves use of laser vibrometers, i.e. optical microphone technology that does not require auxiliary mechanical components. Instead, a beam of light, such as a laser, is split into two parts, one which forms a reference beam and the second which forms a sensing beam which impacts the target surface, e.g., the pressure-sensing diaphragm, and is reflected therefrom, the sensing beam. The sensing beam is heterodyned with the reference beam to produce a phase modulated signal, an interference pattern. This interference pattern models the surface displacement of the target surface, is converted via, an optical interferometer, i.e., a Michelson interferometer, and photodetectors, i.e., photodiodes, to generate a usable, alternating electric current, which mimics the motion/vibration of the target surface, i.e., the pressure-sensing diaphragm.
A known refinement on the laser vibrometer involves using optical grating-like devices consisting of a structure of interdigitated fingers constructed with semiconductors using processes similar to MicroElectroMechanical Systems (MEMS) technology. Instead of using optical interferometers and photodiodes to determine the diaphragm movement, an optical beam is shone onto the semiconductor MEMS like structure while the back-diffracted light beam intensity is monitored. Movements of the interdigitated fingers cause the back-diffracted light beam intensity to exhibit similar temporal changes and thus by monitoring the diffracted light beam intensity, interpretation of the diaphragm movement can be obtained.
In some state-of-the-art optical microphones, an optical fiber probe is deployed with a pressure-sensing diaphragm attached to the tip thereof. The probe light is projected onto the sensing interface and the back-reflected light is collected by the optical fiber tip and sent to the optical interferometer for signal retrieval. In such approaches, the detection sensitivity is very limited due in part to the fact that the aperture of optical fiber is generally very limited, especially for the single-mode fiber that is needed for the said fiber-optic microphones to avoid the generation of higher order modes that would diminish the detected signal output. As a result, the probe light beam must be projected onto the pressure-sensing interface within a very tight angle from normal incidence. This means that the probe light beam can only interrogate the pressure-sensing interface once and hence no possibility of further boosting up the detected signal strength.
Frequently, the detection, resolution and analysis of pressure waves from very weak acoustic signals is required, such as detection of molecules emitted from certain explosives and detection of submerged submarines. In general, optical microphones suffer from limited sensitivity and scalability of output which limits their applicability to analysis of such weak signals. This limited sensitivity results from use of optical interferometers for the detection mechanism, wherein the wavelength of the light beam involved is used as a gauge to monitor the scale of movement of the pressure-sensing diaphragm. Because the optical light sources have a wavelength of approximately 1 micrometer, it becomes increasingly difficult to detect diaphragm movements in scales smaller than 1 nanometer (10−9 meter). Further, with weak signals and longer standoff distances, i.e., the distance between the source and sensing interface or diaphragm, it may become necessary to detect diaphragm movements in the order of 1 picometer (10−12 meter). In fact, for the above examples, involving very weak pressure waves, at distances in the tens of meters away from the diaphragm, it is necessary to detect vibrations of the diaphragm even less than 1 picometer (10−12 meter).